xenobiotic compound

1. BIODEGRADATION OF
XENOBIOTICS
HYDROCARBONS, PLASTICS &
PESTICIDES
DONE BY
SUSHMITA PRADHAN
II SEM M.Sc Microbiology

2. XENOBIOTICS
• It is derived from a greek word “XENOS” meaning ‘foreign or strange’.
• Xenobiotics are those chemicals which are man-made and do not
occur naturally in nature.
• They are usually synthesized for industrial or agricultural purposes e.g.
aromatics, pesticides, hydrocarbons, plastics , lignin etc.
• They are also called RECALCITRANTS as they can resist degradation
to maximum level.

3. BIODEGRADATION
• According to the definition by the International Union of Pure
and Applied Chemistry, the term biodegradation is “Breakdown
of a substance catalyzed by enzymes in vitro or in vivo.
• In other words, defined as the ability of microorganisms to
convert toxic chemicals (xenobiotics) to simpler non-toxic
compounds by synthesis of certain enzymes
• Biodegradation of xenobiotics can be affected by substrate
specificity, nutrition source, temperature, pH etc.

4. SOURCES OF XENOBIOTICS
1. Petrochemical industry :
-oil/gas industry, refineries.
- produces basic chemicals e.g. vinyl chloride and benzene
2. Plastic industry :
- closely related to the petrochemical industry
- uses a number of complex organic compounds
-such as anti-oxidants, plasticizers, cross-linking agents

5. 3. Pesticide industry :
- most commonly found.
-structures are benzene and benzene derivatives,
4. Paint industry :
- major ingredient are solvents,
- xylene, toluene, methyl ethyl ketone, methyl
5. Others :
- Electronic industry, Textile industry, Pulp and Paper industry,
Cosmetics and Pharmaceutical industry, Wood preservation

6. BIODEGRADATION OF PESTICIDES
• Pesticides are substances meant for destroying or mitigating any pest.
• They are a class of biocide.
• The most common use of pesticides is as plant protection products
(also known as crop protection products).
• It includes: herbicide, insecticide, nematicide, termiticide,
molluscicide, piscicide, avicide, rodenticide, insect repellent, animal
repellent, antimicrobial, fungicide, disinfectant, and sanitizer.

7. DIFFERENT METHODS - 4
a) Detoxification:
 Conversion of the pesticide molecule to a non-toxic compound.
 A single moiety in the side chain of a complex molecule is
disturbed(removed), rendering the chemical non-toxic.
b) Degradation:
 Breakdown or transformation of a complex substrate into
simpler products leading to mineralization.
 E.g. Thirum (fungicide) is degraded by a strain of
Pseudomonas and the degradation products are dimethylamine,
proteins, sulpholipids, etc (Raghu et al., 1975).

8. c) Conjugation (complex formation or addition reaction):
 An organism makes the substrate more complex or combines the pesticide
with cell metabolites.
 Conjugation or the formation of addition product is accomplished by those
organisms catalyzing the reaction of addition of an amino acid, organic
acid or methyl crown to the substrate thereby inactivating the pestcides
d) Changing the spectrum of toxicity:
 Some pesticides are designed to control one particular group of pests, but
are metabolized to yield products inhibitory to entirely dissimilar groups of
organisms, for e.g. the fungicide PCNB is converted in soil to chlorinated
benzoic acids that kill plants.

9. There are many mechanisms involved on the biodegradation of pesticides and
other contaminants. These may be summarised as follows:
Dehalogenation- nitrofen, DDT, cyanazine, propachlor.
Deamination- fluchloralin
Decarboxylation- DDTc, biofenox, dichlorop-methyl
Methyl oxidation- bromacil
Hydroxylation- benthiocarb, bux insecticide

10. BIODEGRADTION OF PLASTICS
• Plastic is a broad name given to different polymers with high molecular
weight, which can be degraded by various processes.
• The biodegradation of plastics by microorganisms and enzymes seems to
be the most effective process.
• It consist of two steps- fragmentation and mineralization. But at the core,
reaction occurring at molecular level are oxidation and hydrolysis.
• The decomposition of major condensation polymers (e.g. polyesters and
polyamides) takes place through hydrolysis, while decomposition of
polymers in which the main chain contains only carbon atoms (e.g.
polyvinyl alcohol, lignin) includes oxidation which can be followed by
hydrolysis of the products of oxidation.

12. METHOD
HYDROLYSIS-
 The process of breaking these chains and dissolving the polymers into smaller
fragments is called hydrolysis. E.g. Pseudomonas sps
 Polymeric Chains is broken down into constituent parts for the energy potential
by microorganisms. Monomers are readily available to other bacteria and is used.
 Acetate and hydrogen produced is used directly by methanogens. Other molecules,
such as volatile fatty acids (VFAs) with a chain length greater than that of acetate
is first catabolized into compounds that can be directly used by methanogens.
ACIDOGENESIS-
 This results in further breakdown of the remaining components by acidogenic
(fermentative) bacteria into ammonia, ethanol, carbon dioxide, and hydrogen
sulfide. E.g Streptococcus acidophilus.

13. ACETOGENESIS-
 Simple molecules created through the acidogenesis phase are further
digested by Acetogens to produce largely acetic acid, as well as carbon
dioxide and hydrogen.
METHANOGENESIS-
 Here, methanogens use the intermediate products of the preceding stages
and convert them into methane, carbon dioxide, and water.
 These components make up the majority of the biogas emitted.
 Methanogenesis is sensitive to both high and low pHs and occurs between
pH 6.5 and pH 8. The remaining, indigestible material the microbes cannot
use and any dead bacterial remains constitute the digestate.

14. Some of the microorganism that can degrade plastics are:-
Aliphatic Polyesters
 PolyEthylene Adipate (PEA)- lipases from R. arrizus, R. delemar, Achromobacter sp.
and Candida cylindracea
 Poly (β-Propiolactone) PPL - estereases from Acidovorax sp., Variovorax
paradoxus, Sphingomonas paucimobilis.
Aromatic Polyesters
 Poly-3-Hydroxybutyrate (PHB) – estereases from Pseudomonas
lemoigne, Comamonas sp. Acidovorax faecalis, Aspergillus fumigatus
 Poly Lactic Acid (PLA) - proteinase K from Tritirachium album, Amycolatopsis sp
Strains of Actinimycetes has been reported to degrade polyamide (nylon),
polystyrene, polyethylene.

15. BIODEGRADATION OF
HYDROCARBONS
• A hydrocarbon is an organic compound consisting entirely of hydrogen
and carbon.
• The majority of hydrocarbons found on earth naturally occur in crude
oil.
• Aromatic hydrocarbons (arenes), alkanes,
alkenes, cycloalkanes and alkyne-based compounds are different types
of hydrocarbons.

16. BIODEGRADATION OF PETROLEUM
Petroleum compounds are categorized into 2 groups
1. Aliphatic hydrocarbon e.g. alkane, alcohol, aldehyde
2. Aromatic hydrocarbon e.g. benzene, phenol, toluene, catechol
Aromatic hydrocarbons are degraded aerobically and anaerobically.

17. AEROBIC DEGRADATION
• Are metabolized by a variety of bacteria, with ring fission.
• Accomplished by mono- and dioxygenases.
• Catechol and protocatechuate are the intermediates.
• Mostly found in aromatic compound degradation pathway.

19. 19
OTHER MECHANISMS
1) Photometabolism : in bacteria, this light-induced
“bound oxygen” (OH•
) is used to oxidize substrates

20. 20
2) under nitrate-reducing condition : Nitrate-reducing bacteria
couple the oxidation of organic compound with water to the
exergonic reduction of nitrate via nitrite to N2..
3) dissimilation through sulfate respiration: Sulfate- reducing
bacteria couple the oxidation of organic compound with
water to the exergonic reduction of sulfate via sulfite to
sulfide.

21. 21
Some microorganisms involved in the biodegradation of
hydrocarbons
Organic Pollutants Organisms
Phenolic Achromobacter, Alcaligenes,
compound Acinetobacter, Arthrobacter,
Azotobacter, Flavobacterium,
Pseudomonas putida
Candida tropicalis
Trichosporon cutaneoum
Aspergillus, Penicillium
Benzoate & related Arthrobacter, Bacillus spp.,
compound Micrococcus, P. putida

22. 22
Organic Pollutants Organisms
Hydrocarbon E. coli, P. putida, P. Aeruginosa, Candida
Surfactants Alcaligenes, Achromobacter,
Bacillus, Flavobacterium,
Pseudomonas, Candida
Pesticides P. Aeruginosa  DDT
B. sphaericu  Linurin
Arthrobacter, P. cepacia  2,4-D
P. cepacia  2,4,5-T , Parathion

23. 23
Genetic Regulation of Xenobiotic Degradation
plasmid-borne
mostly in the genus Pseudomonas
PLASMID SUBSTRATE
TOL Toluene, m-xylene, p-xylene
CAM Camphor
OCT Octane, hexane, decane
NAH Napthalene
pJP1 2,4-Dichlorophenoxy acetic acid
pAC25 3-Chlorobenzoate
SAL Salicylate

24. POLYCYCLIC AROMATIC HYDROCARBONS
(PAH)
• Bacteria, fungi, yeasts, and algae have the ability to metabolize both lower
and higher molecular weight PAHs found in the natural environment.
• Most bacteria have been found to oxygenate the PAH initially to form
dihydrodiol with a cis-configuration, which can be further oxidized to
catechols.
• Most fungi oxidize PAHs via a cytochrome P450 catalyzed mono-oxygenase
reaction to form reactive arene oxides that can isomerize to phenols.
• White-rot fungi oxidize PAHs via ligninases (lignin peroxidases and
laccase) to form highly reactive quinones.

25. 25

26. 26
Compound Organisms Metabolite
Naphthalene Acinetobacter calcoaceticus ,
Alcaligenes denitrificans,
Mycobacterium sp. , Pseudomonas sp.,
Pseudomonas putida ,
Naphthalene cis -1,2 – dihydrodiol,
1,2 – dihydroxynaphthalene,
2 - hydroxychromene - 2 – carboxylic
acid, trans – o – hydroxybenzylidene
pyruvic acid, salicylaldehyde, salicylic
acid, catechol, gentisic acid,
naphthalene trans – 1,2 – dihydrodiol .
Acenaphthene Beijerinckia sp., Pseudomonas putida,
Pseudomonas fluorescens,
Pseudomonas cepacia
1- Acenaphthenol, 1- acenaphthenone,
acenaphthene – cis – 1,2 – dihydrodiol,
1,2 – acenaphthenedione,
1,2 – dihydroxyacenaphthylene,
7,8 – diketonaphthyl – l – acetic acid,
1,8 – naphthalenedicarboxylic acid,
3 – hydroxyphthalic acid .
Bacterial strain degrading

27. 27
Compound Organisms Metabolite
Fluoranthene Alcaligenes denitrificans ,
Mycobacterium sp. ,
Pseudomonas putida ,
Pseudomonas paucimobilis,
Pseudomonas cepacia ,
Rhodococcus sp.
7- Acenaphthenone, 1- acenaphthenone,
7- hydroxyacenaphthylene, benzoic acid,
phenylacetic acid, adipic acid,
3- hydroxymethyl – 4,5- benzocoumarin,
9- fluorenone – 1 – carboxylic acid,
8- hydroxy – 7- methoxyfluoranthene,
9- hydroxyfluorene , 9- fluorenone,
phthalic acid, 2- carboxybenzaldehyde
Pyrene Alcaligenes denitrificans ,
Mycobacterium sp. ,
Rhodococcus sp.
Pyrene cis - and trans - 4,5 – dihydrodiol,
4 – hydroxyperinaphthenone, phthalic acid, 4-
phenanthroic acid, 1,2 - and 4,5 –
dihydroxypyrene, cinnamic acid, cis – 2 –
hydroxy – 3 – ( perinaphthenone -9-yl ) propenic
acid
Chrysene Rhodococcus sp. None determined
Benz [a]
anthracene
Alcaligenes denitrificans ,
Beijerinckia sp. ,
Pseudomonas putida
Benz [a] anthracene cis – 1,2, cis- 8,9-, and cis
– 10,11- dihydrodiols, 1- hydroxy – 2 –
anthranoic acid, 2- hydroxy – 3 – phenanthroic
acid, 3- hydroxy – 2 – phenanthroic acid .
Benz [a]
pyrene
Beijerinckia sp.,
Mycobacterium sp.
Benz [a] pyrene cis -7,8 - and cis -9,10 –
dihydrodiols .

28. POLYCHLORINATED BIPHENYLS (PCBs)
• Synthesized chemicals from petro-chemical industry used as lubricants
and insulators in heavy industry.
• First manufactured in 1929 by Monsanto.
• Manufacture and unauthorized use banned in 1978 by USEPA
• Used because-
• Low reactivity
• Non-flammable
• High electrical resistance
• Stable when exposed to heat and pressure
• Used as Hydraulic fluid, Casting wax, Carbonless carbon paper,
Compressors, Heat transfer systems, Plasticizers, Pigments, Adhesives,
Liquid cooled electric motors, Fluorescent light.

29. RISKS-
 Causes reproductive disabilities in animals, human, birds.
 Carcinogenic
 Bioaccumulation
 Soluble in almost all the solvents, fats, oils
 Nervous system damage
 Endocrine gland malfunction

30. METHODS FOR PCB REMOVAL
• Natural Attenuation: Microbes already in the soil are allowed to
degrade as they can naturally and the site is closely monitored.
• Biostimulation: Microbes present in the soil are stimulated with
nutrients such as oxygen, carbon sources like fertilizer to increase
degradation.
• Bioaugmentation: Microbes that can naturally degrade PCB’s are
transplanted to the site and fed nutrients if necessary.

31. PATHWAYS FOR PCB REMOVAL
FUNGAL DEGRADATION –
• Aspergillus niger: fillamentous with cytochrome p450 that attacks
lower chlorinated PCB’s
• Phanerochaete chrysosporium: White rot fungi can attack lignin (PCB)
at low concentration with the help og ligninases.
BACTERIAL DEGRADATION-
• Soil bacteria breaks down PCBs via dioxygenase pathways.
• Most identified seem to be Pseudomonas species, Achromobacter,
Acinetobacter, Alcaligenes, Arthrobacter, Corynebacterium,
Rhodococcus, Burkholderia .

32. REFERENCE
BOOKS-
Sullia S.B and Shantharam S.; General microbiology, Second
edition
Page No. 348-350
Dubey R. C and Maheshwari D.K; A textbook of microbiology,
second revised edition 2009; Page No. 832-836
ARTICLES
Biodegradation of polymers- Dr Rolf Joachim Miller
Biodegradation of pesticides- Andre Luiz Porto, Marcia Nitcshke
and Gleiseda Melgar
Microbial Degradation of Petroleum Hydrocarbon -Nilanjana Das
and Preethy Chandran